For the first time since 2009, the coastline of Lake Superior has frozen hard enough that people can venture out onto the ice and into the sea caves that line the shore near Wisconsin's Apostle Islands. Like the Lake, itself, the sea caves are frozen and covered with sparkling icicles — from dainty needles to thick, massive stalactites.

These are different caves from the ones I went through in a dinghy in the summer a couple of years ago. Those caves were at Devil's Island, about 6 miles from the mainland. The caves you can see in this, and several other videos taken by YouTuber Shannon Kowalski, are right up along the mainland shore, at the base of some steep sandstone cliffs. The cliffs themselves are the remains of a sandy river basin and chains of shallow ponds that dotted the landscape here a billion (yes, with a "b") years ago. The caves are much more recent, forming as waves from Lake Superior slowly erode holes in the sandstone.